The invention relates generally to RF amplifiers and more particularly to an amplifier and method having a dynamically adjustable bias for improving amplifier efficiency.
Traditionally, a semiconductor RF signal power amplifier (PA) is biased with VCC voltage or VDD voltage along with base or gate voltage to establish an operating point for an amplifying transistor or transistor array. In the context of a bipolar transistor, the base voltage or current is established using a bias circuit that is designed to supply a bias point. As is well known in the art, the bias point determines many of the amplifying characteristics of the transistor. In fact, classification of amplifiers into class A, class B, class AB, etc. is primarily determined by the bias point.
Power amplifiers, particularly those based on semiconductor transistors, usually comprise one or more stages of amplification. For example, a first stage of amplification might receive a native RF signal and amplify that signal to a level suitable for a second stage. Typically, three stages of amplification are used in an amplifier suitable for 802.11a, 802.11b, 802.11g, 802.16e, and other wireless standards in order to provide 25 to 35 dB of gain. Great care is taken to set the operating point for each stage of amplification—for each transistor in the chain—such that an optimum gain and linearity are established for each stage, and the complete PA operates in a harmonious manner. For example, the first stage could provide 10 dB of gain, the second stage is biased to provide only 5 dB of gain, and a third and final stage provides 15 dB of gain resulting in a 30 dB amplifier. In addition to an operating point for each transistor in the amplification chain, the matching circuit between transistor amplification stages is designed and manufactured such that an efficient transfer of signal power is supported from stage to stage.
Adjustment of the bias point is a mechanism for amplifier control. In prior art circuits, adjustable bias circuits are provided to allow for circuit tuning—correction of bias current variability caused, for example, by manufacturing inaccuracies. It is well appreciated in the art of amplifier design, that through the operating point, the amplifying qualities of the semiconductor transistor are adjustable such that more or less gain is provided, more or less distortion is manifested, or quite simply more or less current consumption occurs at the operating point. Thus, even within the strict tolerances of semiconductor manufacturing, small post production tuning of a fixed bias may result in significant improvement of the amplifier operation.
Adjustment of the interstage matching circuit is another mechanism for amplifier control. In fact, the matching circuit following the output port of the final stage of amplification is very important in that it matches an output signal from the amplifier to an antenna or load. A mechanism to control the matching circuit at the output port may permit matching for variations in load and protection of semiconductor transistors from large mismatch effects. Thus, for example, external matching circuits as manifested using the placement of L, R, and C components on a printed circuit board are often used to ensure accurate impedance matching between an amplifier and a load.
The operating point and matching circuit deterministically result in a PA providing an amplified RF signal with certain qualities. For example in typical applications, an amount of distortion and power level are two qualities that must be compliant with certain standards established by governmental regulators and/or interoperability committees which have the mandate to certify products as compliant. For example, the IEEE 802.11g standard requires certain qualities in respect of the spectral output of the RF transmission system (Mask) such that signal energy in certain bands is below both relative and absolute threshold values. Moreover, users of such transmission systems are expecting signal clarity resulting from a low probability of transmission error and a low packet error rate. In this respect, error vector magnitude (EVM) as a percentage is used as a figure of merit for signal clarity. For example, a 3% EVM specification might be expected or required from the transmission system in order to achieve a requisite de minimis packet rate. Both compliance with the Mask and EVM are determined by the RF signal quality at an output port of the amplifier, which in turn is determined by the operating point, matching circuit and other settings. Thus, during manufacture or assembly, the EVM is determinable and those tunable portions of the circuit are adjusted to provide a desired EVM and then fixed for the circuit to ensure EVM compliance.
Moreover, it should also be understood that the Mask might vary from jurisdiction to jurisdiction. For example, Mask requirements in Japan are different from those in Europe or North America. Even allowable in-band power output levels might be different in each jurisdiction and expectations in respect of EVM also vary from user to user of the transmission system. A universal goal however, exists from user to user and from jurisdiction to jurisdiction—that is, low power consumption and high power efficiency. Low power consumption will provide the user of a portable transmission device, such as a cell phone, with a long period of usage between charging times. High power efficiency provides the user with less waste of operating energy in the transmission device—getting the most out of the available battery energy. Such waste often manifests as heat generation, which must be dealt with and dissipated. Clearly, using battery power in a wasteful manner is not conducive to long ‘talk time’ in the context of a portable transmission device and generating excess heat is not conducive to comfort if, for example, a battery operated device were to heat up.
A well known trade-off between linear performance (low EVM and compliance with Mask) in an amplifier and power consumption exists. An amplifier can be biased such that it provides very linear amplification performance (low signal distortion) but only at the cost of high power consumption. Therefore, by knowing in advance the linearity requirements, a designer can approximately optimize power consumption to provide just enough linear performance whilst minimizing power consumption.
Traditionally, the operating point and the interstage matching circuits are established and fixed by design. Clearly, if the operating point for each transistor amplifier is fixed at the time of manufacture then an operating point that provides for the most stringent requirements is fixed and the transmission system has no flexibility to adjust bias or match in the context of a varying linearity or power output requirement. Since many country markets (also known as operating jurisdictions) have different requirements, either some of those operating jurisdictions are serviced with devices have less efficient battery usage than is possible or use devices that are customized for those markets. This is clearly less than ideal and it would be efficient to be able to manufacture devices that are capable of serving more than one operational jurisdiction whilst being compliant with the different requirements of that jurisdiction.
In the context of wireless LAN (WLAN) Power Amplifiers (PAs) operating under one of the IEEE 802.11 standards, PAs are typically biased and optimized for best EVM performance at a specified output power. Moreover, the transmit power level for higher modulation rates may be EVM limited depending upon the PA design. As modulation rates are decreased, transmit power can be increased until Mask or Band Edge performance limits the transmit power.
In addition, WLAN PAs are typically designed for class AB operation with fixed bias currents for each individual amplifier stage. Some applications permit the reduction of transmit power to reduce DC power consumption and improve battery life for portable applications. However, simply reducing the transmit power level is inefficient. For example, if the specified performance of the power amplifier is 3% EVM at 20 dBm output power, then backing off the transmit power to 10 dBm will result in very good EVM and Mask performance but at much reduced efficiency because the amplifier is biased to provide a much higher level of linear output power. In contrast, reducing the bias at lower transmit power will drastically improve efficiency and greatly improve battery life.
In AGC design it has been proposed in U.S. Pat. No. 6,763,228 to use a feedback system for controlling bias. The system aims to achieve automatic gain control through bias control, which is achievable since it is known to use bias to control amplifier gain. In U.S. Pat. No. 6,873,211, bias is used to switch an amplifier between a linear mode of operation and a saturated mode of operation. Each of these bias control methods addresses an issue relating to controlling amplification of the amplifier—the bias is changed to control the amplifier's gain—and is controlled in dependence upon gain characteristics of the amplifier.
Further, for RF PAs operating in the range of 2.4 to 10 GHz, digital control is not typically used. These PAs are often based on semiconductor technology platforms such as the Gallium Arsenide (GaAs) compound, which do not feature complementary logic devices and, as such, do not receive or logically manipulate digital signals. In fact, when control signals are required, GaAs PAs work with analog control signals. Further, these same PAs are precluded form working with digital signals especially at lower voltages (e.g. 2.5V, 1.8, 1.2V logic).
It would be advantageous to provide an amplifier and method for improving battery life by improving amplifier efficiency in the context of a variety of operational jurisdictions while maintaining other operating characteristics of said amplifier.